Vortex circulation control in mesoscopic ring magnets

We present a simple method to control the direction of the circulation of t he magnetization in mesoscopic ring magnets, using a uniform magnetic field only. The method is based on the nucleation free switching which occurs wh en the rings switch from the near-saturated state, referred to as the "onio n state," to the flux-closed vortex state. Two possible onion states, forwa rd or reverse magnetized, are possible for a given direction of the magneti c field. Going from the forward or the backward onion state, both local sca nning Kerr microscopy measurements and micromagnetic simulations show that the clockwise or the counterclockwise vortex state, respectively, can be se lected due to asymmetric pinning of the two domain walls that are present i n the onion state. (C) 2001 American Institute of Physics.